1.Technical Field
The present disclosure relates to a selective catalyst reduction (SCR) system that reduces nitrogen oxides (NOx) in exhaust gas and, more particularly, to an SCR system that purifies exhaust gas discharged from a vessel. The present invention is applicable to any kind of vessel and has been developed to satisfy a variety of regulations.
2.Description of the Related Art
An SCR system is used to reduce nitrogen oxides (NOx) in exhaust gas generated during operation of a vessel. For example, the SCR is provided for the purpose of reducing nitrogen oxides discharged from a vessel engine or a boiler in an LNG carrier. The SCR system is based on a selective catalyst reduction method and is configured to reduce NOx into nitrogen and vapor through reaction with a reducing agent while forcing the reducing agent and exhaust gas to pass through catalysts at the same time. In recent years, various attempts have been made to improve the SCR system for vessels, but it is still difficult to achieve satisfactory results due to unique conditions under which such vessels operate, for example, movement and vibration at sea and large loads resulting therefrom.
Conventionally, the SCR system employs NH3 and urea as reducing agents for reducing NOx. Further, the SCR system employs a high temperature catalyst that is activated at 330˜400° C. Thus, the conventional SCR system includes a reheating system for heating exhaust gas at a leading end of a casing of an SCR reactor, which receives a catalyst therein, in order to increase reaction efficiency corresponding to the activation temperature of the catalyst. Further, in the conventional SCR system, a dust collector is provide to a rear end of the casing of the SCR reactor to reduce the amounts of particle materials, particulates, fine dust, and the like in the exhaust gas, which has finished reaction in the SCR reactor.
The reheating system and/or the dust collector cause an increase in the overall size of the SCR system. Hence, the provision of the conventional SCR system to a relatively narrow vessel is very disadvantageous since it results in a further reduction in available space in the vessel. Further, it is necessary for the SCR system to be designed in consideration of conditions of a vessel to which the SCR system will be provided, but the provision of the reheating system and/or the dust collector can make it difficult to obtain an overall design of the SCR system, which must be made in consideration of the conditions of the vessel. In addition, the reheating system and/or the dust collector cause a significant increase in installation cost for the SCR system and require management costs therefor.
Further, since it is necessary for the conventional SCR system to use high-pressure air when supplying a reducing agent such as urea or NH3 into the SCR reactor, the conventional SCR system requires complex piping or installation to guide the high-pressure air to a line supplying the reducing agent and to use a compressor. Moreover, the conventional SCR system has a limit in supplying the urea or NH3 at a high purity into the SCR reactor.
In particular, since the conventional SCR system for vessels cannot stop operation of an exhaust gas source, for instance, a boiler or an engine, upon an abnormal or emergency state, it is impossible or difficult to repair or replace an abnormal part, a broken part or an inoperative part without stopping the operation of the exhaust gas source.